diff --git a/vm/callbacks.cpp b/vm/callbacks.cpp
index 0f7bdfb940..784352b722 100644
--- a/vm/callbacks.cpp
+++ b/vm/callbacks.cpp
@@ -2,12 +2,21 @@
 
 namespace factor {
 
-callback_heap::callback_heap(cell size, factor_vm* parent)
-    : seg(new segment(size, true)), here(seg->start), parent(parent) {}
+callback_heap::callback_heap(cell size, factor_vm* parent) {
+  seg = new segment(size, true);
+  if (!seg)
+    fatal_error("Out of memory in callback_heap constructor", size);
+  allocator = new bump_allocator<code_block>(size, seg->start);
+  this->parent = parent;
+
+}
 
 callback_heap::~callback_heap() {
+  delete allocator;
+  allocator = NULL;
   delete seg;
   seg = NULL;
+
 }
 
 void factor_vm::init_callbacks(cell size) {
@@ -61,18 +70,14 @@ code_block* callback_heap::add(cell owner, cell return_rewind) {
   cell size = array_capacity(insns.untagged());
 
   cell bump = align(size + sizeof(code_block), data_alignment);
-
-  if (here + bump > seg->end) {
+  if (allocator->here + bump > allocator->end) {
     parent->general_error(ERROR_CALLBACK_SPACE_OVERFLOW,
                           false_object,
                           false_object);
   }
 
-  free_heap_block* free_block = (free_heap_block*)here;
-  free_block->make_free(bump);
-  here += bump;
-
-  code_block* stub = (code_block*)free_block;
+  code_block* stub = allocator->allot(bump);
+  stub->header = bump | 1;
   stub->owner = owner;
   stub->parameters = false_object;
   stub->relocation = false_object;
@@ -109,12 +114,14 @@ struct callback_updater {
   explicit callback_updater(callback_heap* callbacks)
       : callbacks(callbacks) {}
 
-  void operator()(code_block* stub) { callbacks->update(stub); }
+  void operator()(object* stub) {
+    callbacks->update((code_block*)stub);
+  }
 };
 
 void callback_heap::update() {
   callback_updater updater(this);
-  each_callback(updater);
+  parent->each_object(allocator, updater);
 }
 
 /* Allocates memory */
diff --git a/vm/callbacks.hpp b/vm/callbacks.hpp
index 385fb680cd..3bb0f6cc82 100644
--- a/vm/callbacks.hpp
+++ b/vm/callbacks.hpp
@@ -25,7 +25,7 @@ from the callback heap in the previous session when the image was saved. */
 
 struct callback_heap {
   segment* seg;
-  cell here;
+  bump_allocator<code_block>* allocator;
   factor_vm* parent;
 
   callback_heap(cell size, factor_vm* parent);
@@ -47,19 +47,6 @@ struct callback_heap {
   code_block* add(cell owner, cell return_rewind);
 
   void update();
-
-  code_block* next(code_block* stub) {
-    return (code_block*)((cell)stub + stub->size());
-  }
-
-  template <typename Iterator> void each_callback(Iterator& iter) {
-    code_block* scan = (code_block*)seg->start;
-    code_block* end = (code_block*)here;
-    while (scan < end) {
-      iter(scan);
-      scan = next(scan);
-    }
-  }
 };
 
 }
diff --git a/vm/slot_visitor.hpp b/vm/slot_visitor.hpp
index 6096c8fb03..1726ac7921 100644
--- a/vm/slot_visitor.hpp
+++ b/vm/slot_visitor.hpp
@@ -206,12 +206,15 @@ template <typename Fixup> struct callback_slot_visitor {
                         slot_visitor<Fixup>* visitor)
       : callbacks(callbacks), visitor(visitor) {}
 
-  void operator()(code_block* stub) { visitor->visit_handle(&stub->owner); }
+  void operator()(object* stub) {
+    code_block *block = (code_block*)stub;
+    visitor->visit_handle(&block->owner);
+  }
 };
 
 template <typename Fixup> void slot_visitor<Fixup>::visit_callback_roots() {
   callback_slot_visitor<Fixup> callback_visitor(parent->callbacks, this);
-  parent->callbacks->each_callback(callback_visitor);
+  parent->each_object(parent->callbacks->allocator, callback_visitor);
 }
 
 template <typename Fixup>